Exercise improves areal bone mineral density (aBMD) and is thought to decrease the risk of osteoporosis in older adults. Preventing osteoporosis is of major importance to Veterans since poor bone health can lead to fractures and functional disability. However, not all forms of exercise have the same effect on bone, so investigating the most effective exercises to improve bone strength is warranted. Currently, the exercises shown to influence aBMD typically involve ground reaction forces (GRF), which include activities such as walking and jumping, and joint reaction forces (JRF), such as weight lifting and rowing. Due to the distinct orthopedic challenges that combat Veterans face, GRF may not always be safe, feasible, or comfortable, so it is necessary to determine what effects JRF may have on bone health in this population. In this randomized trial, 50 older women and men, aged 60 to 75 years, will participate in 24 weeks of supervised GRF or JRF exercise at least 3 times a week. The primary aim is to investigate if GRF and/or JRF exercise in older adults lead to changes in bone strength and volumetric BMD (vBMD) at the femoral neck and lumbar spine. The rationale for this proposal is that, although GRF and JRF have both shown benefit to aBMD, cutting-edge bone-specific imaging and analysis techniques will be used to generate novel information regarding the effects of GRF and JRF exercise on bone strength. Quantitative computed tomography will be used to measure vBMD and bone architecture. 3-D image analysis with finite element analysis and statistical shape modeling will provide quantitative measures of changes in bone strength and geometry based on parameters obtained from quantitative computed tomography. These techniques provide a comprehensive assessment of the effects of exercise on the stress and strain that bone can experience before structural failure (fracture). The proposed research is significant because it compares two different exercise strategies on bone sites at high risk for osteoporotic fracture and will establish a means of comparison for a future definitive exercise trial. This research is innovative because it uses state-of-the-art technology to analyze and determine bone strength and bone shape, in addition to bone mineral density. Information gained from this research will help to define the optimal exercise prescription to improve bone strength in aging Veterans.